Though glucose meters play an essential role in self-monitoring of glucose (blood sugar), there is a higher risk of inaccurate readings with a meter than with blood glucose testing performed by a laboratory. Accurate results are essential for effective glucose management. Inaccurate readings can result in unnecessary changes to diabetes management plans and are usually caused by one of two factors:

• Meter malfunction. Meters and strips sometimes malfunction and give false results. Glucose meters should be tested a minimum of once a month. One way to test a machine and strips for accuracy is to use a standard control solution. These solutions have a known concentration of glucose that can be compared to a meter’s results.
  
  Some meters test accuracy with electronic controls. With this method a cartridge or special test strip is placed in the meter and a signal appears to indicate that the meter is working properly.

Patients are advised to take their meter with them for diabetes appointments. When a meter reading is recorded a few minutes before blood is drawn for laboratory tests, the results can be compared for accuracy.

• Human error. Patients who do not properly use their glucose meters can also be responsible for inaccurate readings. To ensure that the user is testing properly, demonstrations should be done periodically in front of a physician or diabetes educator to refresh techniques even by highly experienced patients.

Other common causes of inaccurate readings include:

• Dirty or wet hands. Having dirt and food on fingers can affect meter readings.
  
  
• Sample size. A blood drop that is too small or any additional blood added to the strip after the initial drop can cause inaccurate readings.
  
  
• Calibration. The amount of reagent (chemical) on test strips can vary from batch to batch. To ensure accuracy, most glucose meters must be calibrated, or standardized, with every new batch of test strips. Failing to calibrate can lead to inaccuracy.
  
  
• Dirty meter. Check the meter’s manual for information on how and how often to clean the meter.
  
  
• Expired test strip. Test strips should not be considered accurate past their printed expiration date.
  
  
• Third-party test strips. Third-party or generic test strips are less expensive than the manufacturer’s test strips but may not work well for the meter. Contact the meter’s manufacturer for information on what test strips to use.
  
  
• Alcohol. When alcohol is used to sterilize the sample site, it must dry completely before blood is drawn or it can affect the reading.
  
  
• Abnormal amounts of hematocrit. Hematocrit is the amount of red blood cells in the blood. People with abnormal levels of hematocrit will test higher and lower for blood glucose than those with normal red blood cell amounts.
  
  
• Altitude, temperature and humidity. These environmental factors can have unpredictable effects on glucose results. High humidity can cause moisture to affect the strip. High and low temperatures can negatively affect the meter and the test strips. Test strips should not be removed from their packaging until the patient is ready to use them. The meter and test strips should be stored and handled according to manufacturer instructions.
  
  
• Interfering agents. Uric acid, vitamin C, glutathione (an antioxidant) and many medications (including aspirin, acetaminophen, estrogens, corticosteroids and antidepressants) in the blood can interfere with glucose testing. Each meter’s manual should list any substances that may interfere with testing accuracy.
  
  
• Age of the meter. Electrical components can begin to break down over time and affect precision. Check the meter’s manual for warranty information.
  
  
• Damage to the meter. Dropping a meter or other rough treatment can reduce its accuracy.
  
  
• Batteries. Low batteries in a meter can affect its performance.
  
  
• Technique. Inexperienced and experienced glucose meter users can make mistakes. Patients should review the manual for step-by-step instructions and perform periodic testing demonstrations in front of their diabetes educator.
  
  
• Whole blood vs. plasma. Blood glucose numbers vary on the type of blood being tested. Readings of plasma blood are usually 10 to 15 percent higher than readings of whole blood. Patients should be aware for which type of blood test they are reading results. The user’s manual should explain if the meter displays results as whole blood or plasma.

In rare cases an infection may develop at the puncture site. To minimize this risk, wash hands thoroughly. If blood is being drawn from a site other than the finger, wash that area as well. If soap and water are not available, clean the area with alcohol but allow drying before testing.